ABSTRACT: SCD exhibits increased susceptibility to infection which in turn leads to SCD-specific pathophysiological sequelae. Infection is the leading cause of death in these patients especially in less developed countries where the blood supply is not efficiently screened. Even in the US, transfusion transmitted pathogens can represent a major threat of morbidity and mortality to SCD recipients. EVs are produced by pathogens and host cells in response to infection and carry serve as vehicles for dissimilation of effector molecules in the infected host. Hemolysis and anemia are two common symptoms that accompany many infectious diseases. While particularly true of parasitic diseases that target red cells, like malaria and babesia, these complications are also seen in many other infections. While the current proposal focuses on babesia, a single pathogen, we believe that the impact of pathogen derived EVs on host cells is a common theme in the pathogenesis of many infectious diseases and results from this study can have implications for hemolytic complications caused by other infectious agents. Hemolytic complications in SCD are the major contributing pathology to fatalities seen in these transfusion transmissions, making it imperative to elucidate the reason for the higher degree of hemolysis seen in these infected patients. EVs derived from parasitized cells can alter cell membranes, impact gene expression and modulate signaling pathways to promote hemolysis. Our overall goal is to define mechanisms of extracellular vesicle driven hemolysis during infection in SCD, using Babesia as our model. Hypothesis: Babesia anemia is exaggerated in SCD because EVs accelerate a) bystander RBC destruction and b) exacerbate ineffective erythropoiesis due to the increased susceptibility of SCD to 1) EV-mediated alterations and complement opsonization of bystander RBCs resulting in direct /indirect lysis 2) immune activation by EVs resulting in increased phagocytosis and induction of autoAbs 3) EV mediated inhibition of enucleation of erythroid progenitors in SCD BM erythropoiesis. To gain insight into these molecular and cellular processes, using in vivo and in vitro SCD infection models, we will: 1) Elucidate mechanisms by which EV Uptake mediates direct hemolysis by examining EV mediated alterations in membranes of SCD RBCs that cause hemolysis 2) Elucidate mechanisms of EV modulation of extravascular hemolysis by examining macrophage polarization and TLR signaling pathway activation, assessing the role and mechanism of autoAb induction by EVs and determining the contribution of autoantibody opsonization and/or complement fixation in bystander RBC clearance 3) Elucidate mechanisms by which EV Uptake contributes to dyserythropoiesis by defining the underlying molecular mechanisms for both EV-induced and infection- induced enucleation defects in erythroid progenitor cells in SCD bone marrow. Our proposed studies will result in a better understanding of the role of parasite EVs in the pathogenesis associated with human babesiosis especially the hyper- hemolysis seen in the sickle cell context. We anticipate these studies will lead to the refinement of diagnostic tools and offer novel strategies to combat anemia in human babesiosis.